Polyimide resin, which has excellent heat resistance and outstanding electrical and mechanical properties, has been used for forming surface protection and interlayer insulation films of semiconductor devices. In recent years, however, polybenzoxazole resin has begun to be used because it does not have an imide ring-derived, highly polar carbonyl group and thus shows good moisture resistance reliability. In addition, a photosensitive resin composition has been developed, which comprises a resin provided with photosensitivity and is thus able to simplify a part of the process of forming a relief pattern.
Recently, positive photosensitive resin compositions comprising a polybenzoxazole precursor and a diazoquinone compound (photosensitive agent) have been developed, which can be developed with alkaline aqueous solutions as a result of further improvements in safety (for example, see Patent Literature 1).
Herein, the production of a relief pattern with a positive photosensitive resin composition and the mechanism of developing the same will be described. A coating film of a positive photosensitive resin composition is formed on a wafer and exposed to actinic radiation with an exposure device through a mask, the device being called “stepper”. As a result, a portion of the film which was subjected to the exposure (hereinafter referred to as “exposed portion”) and a portion of the same which was not subjected to the exposure (hereinafter referred to as “unexposed portion”) are formed. The diazoquinone compound which is present in the unexposed portion is insoluble in alkaline aqueous solutions; moreover, it becomes resistant to alkaline aqueous solutions by the interaction with the resin. On the other hand, the diazoquinone compound which is present in the exposed portion causes chemical change by the action of the actinic radiation to produce a carboxylic acid; therefore, it becomes soluble in alkaline aqueous solutions and accelerates the dissolution of the resin. By dissolving and removing the exposed portion using the solubility difference between the exposed and unexposed portions, it becomes able to produce a relief pattern comprising the unexposed portion only.
In the case of using such a photosensitive composition, its sensitivity is very important. When the sensitivity is low, that is, when the sensitivity is poor, it takes a long exposure time, resulting in a low throughput.
Consequently, to improve the sensitivity of the photosensitive resin composition, when the concentration of the photosensitive group (naphthoquinone diazide sulfonyl group) in the photosensitive resin composition is increased by adding a large amount of photosensitive agent (photosensitive diazoquinone compound) or increasing the substitution rate of the photosensitive group in the photosensitive agent, the dissolution rate of the exposed portion is accelerated, that is, the sensitivity is improved. However, there is a problem that an undissolved residue of the photosensitive resin composition (hereinafter referred to as “scum”) is produced at the bottom of the pattern in the exposed portion after development, the residue being derived from, for example, the photosensitive resin composition which comprises the photosensitive agent, etc. which failed to be an alkali-soluble compound since the actinic radiation caused poor chemical change.
A different method for improving the sensitivity is a method in which, while the developing time is lengthened to increase the dissolution amount of the exposed portion in alkaline aqueous solutions, for increasing the resistance of the unexposed portion to alkaline aqueous solutions, the concentration of the photosensitive group in the photosensitive resin composition is increased as described above, the dissolution rate of the unexposed portion is decreased by increasing the molecular weight of the resin, or the molecular weight of a phenolic compound is increased, which is a component that retains or supports the photosensitive agent. However, this method not only results in a low throughput but also increases the scum.
Similarly, when the dissolution rate of the exposed portion is increased by, for example, decreasing the molecular weight of the resin to increase the dissolution amount of the exposed portion in alkaline aqueous solutions, the unexposed portion is also more likely to be soluble; therefore, it is difficult to form a relief pattern and when it collapses, there is a problem such as scum production and a deterioration in resolution. Furthermore, the unexposed portion is needed to have a predetermined film thickness or more, so that the developing time is extremely shortened, and not only is it difficult to control pattern production, but also there is a deterioration in the sensitivity of the photosensitive resin composition, to the contrary.
It is also important to improve resolution because, with an increase in the degree of integration of semiconductor chips in recent years, the size of relief pattern has been declining steadily. It is said that there is a trade-off relationship between resolution and sensitivity; therefore, there is a strong demand for the development of a photosensitive resin composition which produces no scum, shows high sensitivity and high resolution, and has high relief pattern productivity.
A positive photosensitive resin composition is mentioned in Patent Literature 2, which comprises an alkaline soluble resin consisting of an acrylic copolymer, 1,2-naphthoquinone diazide-5-(and/or -4-) sulfonyl ester using a polyhydroxy compound and a crosslink agent, and tetrakisphenol compounds are mentioned as polyhydroxy compound examples (see formulae [I-c] to [I-e] in paragraph 0037 of the literature).
However, the positive photosensitive resin composition of Patent Literature 2 uses an acrylic copolymer as the alkaline soluble resin and uses no polyimide resin or polybenzoxazole resin.